TECHNOLOGY FOR PRODUCTION OF URANIUM OXIDE FOR

Dr. Sheela, Dr. Meena R, Dr. Anuradha, Dr. R.M. Kartha, Dr. R.K. Pujar,
Dr. Ajit Singh, Dr. C. Ganguly, Nuclear Fuel Complex, Hyderabad.

Abstract

Uranium in the form of Uranium Oxide Powder is the main constituent for fabrication of nuclear fuel for the Atomic power reactors for generation of electricity and also for the research purposes. India has two types of commercial Atomic power reactors, namely Pressurized Heavy Water Reactors (PHWR) and the Boiling Water Reactors (BWR). We, at NFC are engaged in the production of nuclear fuel and all the core hardware for these reactors, starting from the uranium concentrate in the form of Magnesium Di-Uranate (MDU) for the PHWRs, UF6 for the BWRs and Zircon sand for the production of zirconium an important constituent of zirconium alloy components. Nuclear grade uranium oxide is produced through various chemical process steps, such as dissolution of the uranium concentrate/ UF6, purification by solvent extraction, precipitation of uranium compound, drying, calcination, reduction, etc for conversion to uranium oxide powder, followed by
metallurgical & mechanical operations such as compaction, high temperature sintering, grinding, cladding, TIG welding, resistance welding etc, for fabrication of the fuel assembly.

The wide variety of processes necessitates advanced analytical and testing techniques for maintaining the high quality of with respect to purity & integrity of the assembly along with close control of parameters at each stage of production. It is also required to ensure safety and environmental protection against chemical & radiological hazards, as well as operational hazards.

This paper discusses the UO2 production technology adopted at NFC. It needs to be mentioned that men & women technologists are working here at equal footing in the above activities.

1.2 India has planned a Three Stage programme for generation of Nuclear Power, under the aegis of Atomic Energy Commission of the country. The First Stage is planned with Pressurised Heavy Water Reactors (PHWR) & Boiling Water Reactors (BWR) with utilization of scarcely available uranium resources of the country for power generation & production of Plutonium. In the Second Stage power will be produced by using plutonium produced in First Stage as driver fuel, along with vast reserves of thorium available in the country in the Fast Breeder Reactors to produce Uranium ₂₃₃. In the third stage power will be produced using the artificially produced Uranium-233 from breeding of Thorium-232 along with U235 & U238 & Pu in the Advanced Heavy Water Reactors.

1.3 As a part of this programme, Nuclear Fuel Complex was set up, under the Department of Atomic Energy for production of Uranium Oxide Fuel Assemblies and all the core components. As mentioned above India has two types of Nuclear Power reactors, namely PHWRs & the BWRs. Fuel Assemblies for both types of reactors are produced at NFC in the Natural Uranium Oxide Fuel Plant and Enriched Uranium Oxide Fuel Plant. In addition, NFC also produces all the reactor core components and structurals of Zirconium alloys for Nuclear Power Reactors of the country

India has planned a Three Stage program for generation of Nuclear Power, under the aegis of Atomic Energy Commission of the Country. The First Stage is planned with utilization of scarcely available Uranium resources for power generation & production of Plutonium in the Pressurized Heavy Water Reactors (PHWR) & Boiling Water Reactors (BWR). Presently India has 12 working reactor of the PHWR type and 2 of BWR type. In addition, 2 reactors of 500 MW with natural uranium fuel are under construction at Tarapur.

Two more reactors of 1000MW capacity each are proposed to procure on turnkey basis from Russia. These reactors will have enriched uranium oxide fuel.

In the Second Stage power will be produced in the Fast Breeder Reactors by using the Plutonium produced in First Stage, along with vast reserves of thorium available in the country. India has vast reserves of Thorium, which is a radioactive material but not fissile. Thorium on irradiation will give Uranium 233 an artificially produced fissile isotope of uranium. Hence, in the Second Stage of power programme the total fuel assembly will comprise of natural uranium-plutonium fuel as driver with thorium blanket for breeding. Hence, stress in the second phase of power programme will be Fast Breeder Reactors. A test reactor of this type already exists in Kalpakkam, Tamil Nadu.

In the Third Stage power will be produced using the artificially produced Uranium-233 from breeding of thorium 232 along with uranium and plutonium available in the Advanced Heavy Water Reactors. Extensive engineering level R & D is being done for this purpose at BARC.

The Nuclear Fuel Complex, a unit of Department of Atomic Energy (DAE) is the product of intensive research and development work carried out at Bhabha Atomic Research Centre, (BARC) during sixties in the field of fabrication of Nuclear Fuel and other reactor-core components required to support Indian Nuclear Power Programme. Nuclear Fuel Complex has facilities for production of Natural Uranium Oxide Fuel for the PHWR and Enriched Uranium Oxide Fuel for BWR in the Fuel Fabrication Plants. In addition, fabrication of zirconium alloy components such as Fuel Tubes, Coolant Tubes, Calendria Tubes, Spacer Pads, End Plugs, End Plates, Tie Plates, Connector Plugs, Garter Springs, etc. is done in the Zirconium plants, starting from Zircon sand, requiring various chemical, metallurgical and mechanical operations. NFC also produces a number of high purity materials in the range of 4N & 5N purity, required for the electronic industry in addition to Nuclear Industry.

In view of radioactive material being handled at NFC and the nature of end use in the Nuclear Reactors, the Fuel Assemblies and all the intermediates products are put through stringent quality control and testing. This is required in order to ensure that no failures occur during the use of these assemblies in the reactors and also to ensure availability of required quantity of uranium (the fissile component) in the reactor for smooth operation and power generation. The various techniques that are used for this purpose include ICP, XRF, XRD, Spectrophotometry, Mass spectrometer, Particle Size analyzer, for checking chemical composition & physical parameters and a number of NDT techniques like gamma scanning, Ultrasonic Testing, Radiography etc. for weld evaluation to check the soundness of the Fuel Assemblies & other components. It may be noted that NFC is an Organisation with ISO9002 Certification.

4.1.1 The starting material for natural uranium oxide fuel used in the PHWR Type of reactors is the indigenously available uranium concentrate mostly in the mineral belt of Jharkhand that is produced at the uranium mines of Uranium Corporation of India Ltd. (UCIL).

The Fuel Assembly for the BWR Type of reactors is made of Enriched Uranium, which is imported in the form of Uranium Hexafluoride.

4.1.2 The conversion of Uranium concentrate/UF₆ to nuclear grade UO₂ involves various chemical process steps such as Dissolution of Uranium Ore Concentrate in nitric Acid to solubilise uranium for the PHWR fuel and hydrolysis of UF₆, in case of BWR fuel production stream.

4.1.3 The crude uranium solution is then subjected to purification by Solvent Extraction in the Slurry extraction setup developed indigenously using TBP- Nitrate system of extraction. This is a unique setup, experimented on pilot plant scale & then designed, fabricated, installed & put into operation on plant scale in the Uranium Oxide Plant of NFC to cater to the fuel requirement of all PHWR type of reactors in the country. This unit operates only with compressed air for material transfer, control of flow rates, mixing, settling etc. In the solvent extraction set-up at NFC, organic phase is the continuous phase with internal recycle of the organic solvent against the normal convention of having aqueous phase as the continuous phase.

4.1.4 The uranium thus obtained in pure solution form is then precipitated with concentrated ammonium hydroxide solution to get Ammonium Diuranate (ADU) Cake. The precipitation is carried out in batch mode at equilibrium conditions with slow addition of ammonium hydroxide and mixing. After completing precipitation, the resultant ADU slurry is filtered and washed simultaneously to produce ADU cake, which is then taken for drying. Precipitation of ADU is the most important step in production of reactor grade UO₂ powder & a close control of all the variables is essential for product quality.

4.1.5 Pure ammonium diuranate cake from batch precipitation is dried to obtain dried ADUC/UO₃ in a Box furnace (Batch operation)/ turbo dryer (Continuous operation)/ Spray dryer (Semi continuous operation). All the three types of units are in use in NFC at the three fuel production plants.

4.1.6 The dried ADUC is collected in leak-proof containers and calcined in the presence of air by passing through electrically heated rotary tubular furnace to get Uranium Oxide (U₃O₈). This oxide is reduced to UO₂ in different electrically heated rotary tubular furnace in cracked ammonia atmosphere (N₂ + H₂). The off gases comprising of water vapour and unused gases (N₂ + H₂) are scrubbed in a water spray column before they are let out. The UO₂ powder thus produced is subjected to partial oxidation on the surface with limited quantity of air in a rotary tube again, to get a stable Uranium Dioxide (UO₂) powder.

4.1.7 The UO₂ powder is then compacted in a hydraulic press/rotary compactor to increase the bulk density, granulated and mixed with binder. The binder-mixed granules are subjected to final compaction to give the cylindrical shaped pellets of specified dimensions & density (Green Pellets).

4.1.8 These Green Pellets are loaded/stacked in molybdenum containers(open)/boats and sintered at high temperature under reducing atmosphere for further densification to achieve min. 96% of the theoretical density of UO₂pellets. To ensure uniform surface finish and diameter, these pellets are centreless ground and inspected for physical integrity, density, purity, surface defects to meet the specifications.

4.1.9 The inspected pellets are loaded in zircaloy tubes after thorough washing and drying and are welded at the both ends by suitable welding techniques. All the required components are then welded on these tubes & the welded elements are assembled in a pre-determined metrics to form the fuel assembly. The BWR assembly comprises of over 260 components and PHWR assembly comprises of 167 components. These fuel assemblies are subjected to a stringent quality control checks using NDT techniques, before final acceptance & clearance. The inspected & accepted assemblies are packed and dispatched to reactor sites.

At NFC, we have three facilities for production of UO2 fuel – namely Enriched Uranium Oxide Plant (EUOP), Uranium Oxide Plant(UOP) & New Uranium Oxide Plant (NUOP).

Of these three plants, EUOP & NUOP are presently being headed by Women Technologists at NFC and even at BARC the Uranium Metal Plant is headed by a Women Technologist (all engineers) to manage production of Uranium Metal Oxide and Uranium Metal for further fabrication of fuel. All three women are chemical engineers & have been consistently giving targeted production .

Apart from this, many women scientists (over 10) are associated with Quality Control and Assurance of Fuel Assembly at various stages of production. These include Post Graduates in physics, chemistry, diploma holders in electronics and instrumentation, etc.

ONVERSION PLANT FLOW SHEET

SCRAP / MDU

UF6 SOLID in CYLINDERS